Create src/app/service.go: the application-service layer that becomes
the single owner of the durable jobs slice and the transient runtime
map, guarded by a non-reentrant sync.Mutex. NewService wires a loaded
store; Open() is the convenience entry point. Read-only accessors
(Jobs/Runtime/Store) take the lock, and Jobs() returns a copy to keep
callers from mutating Service-owned state.
State-mutating intents and the event/observer machinery are deferred to
T3.2-T3.4. Adds no-Fyne unit tests for runtime construction, copy
isolation, and store wiring.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
mixeme